Machines of various sorts include motorized stages including carriages and other slide mechanisms as well as spindles and pivots for effecting relative translational or rotational motions along or about machine axes. The relative motions can be carried out, for example, between tools and workpieces on machine tools or between sensors and test parts on measuring machines.
The motorized stages are supported for relative motion directly on a machine frame or indirectly on other motorized stages. Guide systems include rails, bearings, shafts, ways, and rollers for defining the desired motions. Typical linear drives include belts, rack and pinions, ball screws, and lead screws powered by stepper or servo motors.
Belt drives transform rotary motion to linear motion via a timing belt that typically contains teeth in engagement with a similarly toothed pulley driven directly or indirectly by a drive motor. When implementing a linear belt drive on a vertical (Z-axis) of a machine, a counterbalance is often needed to offset the weight of a vertically displaceable slide mechanism. As such, one end of the timing belt, which is otherwise supported by one or more pulleys attached to the machine frame or other form of mount, is affixed to the vertically displaceable slide mechanism and the other end of the timing belt is affixed to a counterweight. One of the pulleys is a drive pulley connected directly or indirectly through a gear reducer or other power allocator to a rotary drive motor. The mass of the counterweight which is equal or nearly equal to the mass of the vertically displaceable slide mechanism creates a balanced load or nearly balanced load on either side of the drive pulley. The balanced load reduces the amount of holding force that must be exerted by the motor to keep the vertically displaceable slide mechanism in a constant Z-axis location. This reduces the amount of heat created by the motor and can also allow for a smaller less expensive motor to be used.
One problem with this known implementation can arise should the vertically displaceable slide mechanism encounter an unexpected obstruction while being lowered. Despite the presence of a counterweight, the entire weight of the slide mechanism can rest on the obstruction, because by even slightly overdriving the slide mechanism toward its desired position, the drive motor can lift the counterweight while the timing belt goes slack between the drive motor and the slide mechanism. Applying the full gravitational force of the slide mechanism against the obstruction could damage the slide mechanism or harm the obstruction, which could be a part of an operator's body. While control loop monitoring for excess position errors can limit the amount of overdrive, any amount of overdrive can unload the counterbalance transferring undesirable weight to the obstruction.